As an example of liquid ejection apparatuses which eject liquid from nozzles, ink jet printers are known in the art. With regard to print heads for inkjet printers, thermal print heads which eject ink using thermal energy and piezoelectric print heads which eject ink using piezoelectric elements are known in the art.
In thermal print heads, one side of ink cells is covered with a nozzle sheet having small nozzles, and heating elements are disposed in the ink cells. Ink bubbles are generated in the ink cells by rapidly heating the heating elements, and ink drops are ejected from the nozzles by a force applied by the ink bubbles.
FIGS. 15 to 18 are diagrams showing an example of a thermal print head chip a (serial type). FIG. 15 is a perspective view of the print head chip a, and FIG. 16 is an exploded perspective view of FIG. 15 where a nozzle sheet g is shown separately. In addition, FIG. 17 is a plan view showing the detailed relationship between an ink cell b (barrier layer f), a heating element c, and a nozzle h. In FIG. 17, the nozzle h is shown by double-dotted chain lines on the heating element c. In addition, FIG. 18 is a sectional view of FIG. 17 cut along line A—A, where the nozzle sheet g is also shown.
In the print head chip a, a base member d includes a semiconductor substrate e composed of silicon or the like and heating elements c formed on one side of the semiconductor substrate e by deposition. The heating elements c are electrically connected to an external circuit via conductors (not shown) formed on the semiconductor substrate e.
A barrier layer f is composed of, for example, a light-curing dry film resist, and is constructed by laminating the dry film resist on the surface of the semiconductor substrate e, on which the heating elements c are formed, over the entire region thereof and removing unnecessary parts by a photolithography process.
In addition, the nozzle sheet g has a plurality of nozzles h and is formed of, for example, nickel, by using an electroforming technique. The nozzle sheet g is laminated on the barrier layer f such that the nozzles h are positioned in accordance with the heating elements c, that is, such that the nozzles h are positioned directly above their respective heating elements c.
Ink cells b are constructed of the semiconductor substrate e, the barrier layer f, and the nozzle sheet g, such that the ink cells b surround their respective heating elements c. More specifically, in the figure, the semiconductor substrate e serves as the bottom walls of the ink cells b, the barrier layer f serves as the side walls of the ink cells b, and the nozzle sheet g serves as the top walls of the ink cells b. Accordingly, the ink cells b are open at the right front sides thereof in FIGS. 15 and 16, and are communicating with an ink path i via the open sides thereof. Ink is supplied to the ink cells b only through these open sides, and is ejected from the nozzles h, which are the only openings in the ink cells b except for the open sides.
Normally, a single print head chip a includes hundreds of heating elements c and ink cells b containing the heating elements c. The heating elements c are selected in accordance with a command issued by a controller of a printer, and ink contained in the ink cells b corresponding to the selected heating elements c is ejected from the nozzles h.
More specifically, in the print head chip a, the ink cells b are filled with ink supplied via the ink path i from an ink tank (not shown) which is combined with the print head chip a. When a current pulse is applied to, for example, one of the heating elements c for a short time such as 1 to 3 microseconds, the heating element c is rapidly heated, and a bubble of ink vapor (ink bubble) is generated on the surface of the heating element c. Then, as the ink bubble expands, a certain volume of ink is pushed by the ink bubble. A part of the pushed ink returns to the ink path i from the corresponding ink cell b, and another part of the pushed ink is ejected from the corresponding nozzle h as an ink drop. The ink drop ejected from the nozzle h lands on a print medium such as a piece of paper.
In addition, after the ink drop is ejected, ink is supplied to the ink cell b in an amount corresponding to the ejected ink drop before the next ejection. In order to efficiently eject an ink drop instantaneously at the time of ink ejection (at as high a speed as possible), the open sides (area of L1×L2 in FIG. 18) of the ink cells b are preferably as small as possible and a pressure in the ink cells b and the nozzles h at the time of ink ejection is preferably as high as possible. However, in such a case, a path resistance which occurs when ink flows into the ink cells b increases. Accordingly, a long time is required for refilling the ink cells b and a period at which ink ejection is repeated increases.
Accordingly, the ratio of an effective area (Sn) of the open sides of the nozzles h and the area of the open sides of the ink cells b (Si=L1×L2) is set to a suitable value R(═Sn/Si). The ratio R may of course be set to a specific value (depending on the ink-ejection speed, the print precision, the print speed, etc.).
In order to maintain the size and the ejection direction of the ink drops ejected within predetermined ranges, the following conditions must be satisfied:
(1) The sum of the internal volume of the ink cells b and the internal volume of the nozzles h is within a predetermined range;
(2) Even if the pressure inside the ink cells b increases when the ink drops are ejected, the semiconductor substrate e, the barrier layer f, and the nozzle sheet g are reliably adhered to each other and ink leakage does not occur; and
(3) The internal volume of the ink cells b does not change when the ink drops are ejected.
If the resolution is relatively low, such as 300 dpi, the above-described conditions can be satisfied without increasing the processing accuracy. However, when the resolution is increased to, for example, 600 dpi or 1200 dpi, ink ejection performance is affected by the accumulation of processing errors and adhesion errors.
In the above-described print head chip a, since each ink cell b has only one inlet, if this inlet is clogged with, for example, dust mixed in ink, an ink-supply speed at which ink is supplied to the ink cell b decreases and a sufficient amount of ink cannot be supplied. In addition, since the open area of the inlets of the ink cells b is normally greater than the open area of ejection holes of the nozzles h, dust particles which travel into the ink cells b through the inlets thereof cannot always pass through the ejection holes.
Accordingly, there is a risk that the dust particles will remain around the heating elements c. When the dust particles remain on the heating elements c, it becomes difficult to eject ink drops normally. In particular, as the size of the ink drops is reduced to achieve high resolution, the above-described problem becomes more severe. Thus, there is a risk that ink drops of a predetermined volume cannot be ejected and the image will be blurred.
Dust exists at every point along the path of ink. Accordingly, in order to prevent the ejection holes of the nozzles h from being clogged with dust, components which come into contact with ink must be thoroughly cleaned and various kinds of dust-removing filters must be placed at multiple positions.
However, since the amount of ink supplied to the ink cells b increases as the print speed increases, if the meshes of the dust-removing filters are too fine, ink cannot be supplied sufficiently quickly. Even if there is no problem at first, dust will collect on the dust-removing filters over time and it will become difficult for ink to smoothly pass through the dust-removing filters, and eventually, ink cannot be supplied sufficiently quickly. Thus, the print quality will be degraded (for example, the image will be blurred).
The above-described problems also occur in piezoelectric print heads.
The volume of the ink drops ejected closely relates to the internal volume of the ink cells b and that of the nozzles h, and the processing accuracy of these parts must be maintained to maintain the volume of the ink drops constant. In particular, when the volume of each ink drop ejected is large, that is, when the resolution is relatively low, the above-described processing accuracy does not have a large influence. However, when the resolution is high, the volume of ink drops ejected is extremely small, and high processing accuracy is required accordingly. Although this is technically possible, high costs are incurred in order to obtain high processing accuracy.
Accordingly, a technique has been used in which a plurality of ink drops are delivered to the same position (overwrite is performed a plurality of times) to average the ink drops delivered, so that variation caused when the ink drops are ejected and ejection failure due to dust mixed in ink become indiscernible.
Although this process is effective for improving the image quality, even when the volumes of ink drops ejected from the nozzles h and ejection angles thereof are constant and the print head chip a has absolutely no defects, printing is performed more than once and the ink drops are repeatedly delivered to the same position. Therefore, there is a problem that a long printing time is required. This contradicts to the requirements of the market for high print speed.
On the other hand, print heads for line printers in which multiple print head chips a are arranged along a print line and which do not move along the print line during printing are known in the art. In this construction, however, it is difficult to perform overwrite a plurality of times as described above.
As described above, in the known constructions, difficulties regarding processing accuracy and measures against dust are barriers to high-resolution and high-speed printing.